Documentation for RTK-Surveyor contains the following statement about SBAS
“Satellite-based Augmentation System or SBAS is disabled by default. This is because there is a bug in u-blox’s v1.13 firmware for the ZED-F9P that causes the RTK status LED to stop functioning when SBAS is enabled. We have found no major difference in lock times or accuracy with SBAS turned off but SBAS can be enabled if desired. Enabling SBAS will cause the RTK LED to no longer illuminate, regardless of the RTK fix.”
The statement is not entirely true as will be explained in detail in following paragraphs. When RTK is not enabled, SBAS provides correction for tropospheric delay which can result in 1 to 2 meters of additional error if uncorrected. RTK enabled with a reference station within reasonable range also corrects for tropospheric delay. The RTK light is not illuminated with a RTK FLOAT solution, but it is illuminated when the receiver achieves a RTK FIX solution with SBAS enabled.
There are two types of propagation delay from GPS satellites to the GPS receiver. Ionospheric delay is the more significant delay but can be almost entirely removed in a dual frequency receiver such as the UBlox ZED-F9P. Tropospheric delay is introduced in the lower 10 km of the atmosphere and varies with time and location. A dual frequency receiver does not remove this delay; it can only be corrected using a ground reference stations such as the SBAS service.
The ZED-F9P reports an instantaneous error estimated from the received satellite solutions. This error does not include the error from tropospheric delay which varies slowly with time. The only way to measure this error is to place the GPS receiver at a fixed known point and record the position over many hours. A 24 hour measurement is best, but 6 hour time span can give a reasonable estimate if it is done over the transition between day and night. The known position can be measured in RTK FIX mode using a nearby reference station.
The attached figure shows the accuracy results over a 6-hour period. The plot shows the error from the RTK FIX position with SBAS Enabled in blue and SBAS Disabled in red with measurements done from 3 to 9 pm on adjacent days. With SBAS Enabled the tracking is a tight cluster around the zero-error point with a maximum error of 1 meter. With SBAS Disabled, the error wanders over a wide region with a maximum error of 2.5 meters. If RTK cannot be used, the receiver performance is significantly degraded with SBAS Disabled.
With SBAS Enabled using an RTK reference station, the RTK status LED will illuminate when the receiver achieves RTK FIX mode. This should reduce the error to a few cm if the RTK reference station is within 20 km. Stations further away will have a higher error which is not included in the ZED-F9P error calculation and needs to be added by the user.
RTK FLOAT mode will not activate the RTK status LED. This is the correct decision by UBlox since errors in RTK FLOAT mode are unknowable. RTK FLOAT is attempting to solve a complex multidimensional problem and there can be large variations in position during this calculation. Measurements I have made in forested areas in RTK FLOAT have shown some position errors larger than the error measured in the non-RTK SBAS mode.
Users can enable SBAS following the instructions in the RTK Surveyor documentation.
